This invention relates to broadband antennas. It particularly relates to spiral and sinuous antennas of reduced size relative to conventional spiral and sinuous antennas of corresponding bandwidth.
The cavity backed spiral antenna has been used for a number of years as a means of providing circularly polarized radiation over a broad frequency band. The two most popular configurations are the dual arm equiangular and the Archimedean spirals, in which the two arms are fed in antiphase at the center. In both cases the radiating mechanism is the same and the radiation takes place from a region centred on one wavelength in circumference. Clearly, the lowest frequency of operation is determined by the diameter of the spiral, where the outer circumference is equal to the longest wavelength. If space is at a premium, then a square Archimedean configuration may be used to gain an aperture reduction in the ratio of .pi.:4. Further aperture reduction is accomplished, as taught by Morgan in Proc. 9.sup.th European Microwave Conf. September. 1979, pages 181-185, by forming a square spiral with a zigzag track to produce a slow wave structure. However, this approach limits the bandwidth of operation by reducing the resolution of the central region of the spiral, owing to the square characteristics of the geometry. This, combined with the zigzag modulation, results in an ill-defined geometry at the center of the spiral and limits the upper frequency of operation.
"An Introduction to Wideband Two-Channel Direction-finding System" (Microwave Journal, February 1984 pages 91-106, J. A. Mosko) describes an attempt to increase the effective aperture size using a four-arm spiral having sinusoidally-modulated filaments. This was said to have resulted in fairly poor success.
Other attempts to produce dual polarization antennas are disclosed in U.S. Pat. No. 5,227,807. These feature the provision of one or more pairs of quasi-spiral antennas of opposite hand arranged adjacent each other, the spirals being distorted to fit the or all pairs of spirals into a single circular footprint. The quasi-spirals are based on prototype spirals, each having an archemedian inner region and a logarithmic outer region, and one disclosed arrangement has sinuous outer turns to enable the spirals to be packed into the semi-circular areas more efficiently. This proposal uses an abrupt transition between the inner smooth quasi-spiral and the outer modulated spiral.
The sinuous antenna, as taught by DuHamel in European Patent EP-A-0198578, is an alternative form of cavity backed broadband printed antenna which has similar performance to the conventional spiral antenna, but is also capable of dual polarization. The four-arm sinuous antenna has generally sinuous arms extending outwardly from a common point and arranged at intervals of 90.degree. about the central axis. Each antenna arm comprises cells of bends and curves, each cell being interleaved without touching between adjacent cells of an adjacent arm. In its more popular configuration, opposite arms are fed in antiphase, and the phase relationship between orthogonal pairs of arms can be chosen to be either 0.degree. for linear polarization, +/-90.degree. for opposite senses of circular polarization, or some arbitrary angle for elliptical polarization. The mechanism of operation is similar to the conventional spiral. Briefly, a single cell, comprising a pair of bends, will radiate if it is approximately one half wavelength in electrical length. The angular width of a single cell is typically about 90.degree.. Thus the active radiating region at a given frequency will be about one wavelength in circumference. This means that for a minimum frequency of operation, the conventional spiral and the sinuous antenna are of approximately equal size.